The present disclosure relates to a system and method for preventing pressure damage to a fuel cell. In particular, the present disclosure relates to relational and bidirectional water seals for fuel cell pressure balance when processing anode exhaust gas.
In general, a fuel cell includes a negative or anode electrode and a positive or cathode electrode separated by an electrolyte that serves to conduct electrically charged ions between them. A fuel cell will continue to be able to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are provided adjacent to the anode and cathode through which fuel and oxidant gas are supplied. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each fuel cell and an adjacent fuel cell.
In high temperature fuel cell stacks, fresh air usually serves as oxidant and is provided at the entry of the cathode-side of the fuel cell stack. This fresh air is typically at ambient temperature and must be heated to the operating temperature of the fuel cell stack. Conventionally, unused fuel in the anode exhaust gas exiting from the anode-side of the fuel cell stack is oxidized, or burned, with the incoming fresh air to heat the air. In order to ensure complete reaction of fuel and to minimize temperature gradients, the anode-exhaust must be completely mixed with air.
During operation of the fuel cell stack, at a junction of the two process gas streams, gas pressure at the exit of the anode-side of the fuel cell stack is coupled to gas pressure at the inlet of the cathode-side of the fuel cell stack. Typically, the pressure at the exit of the anode-side is necessarily higher than the pressure at the inlet of the cathode-side by an amount required to overcome pressure losses associated with any connection piping and with the oxidizer used to burn the anode exhaust and incoming oxidant gases. An anode exhaust processing system may be added to address both the gas mixing and the pressure differential problems. The anode exhaust processing system may include, for example, a mixer-eductor-oxidizer (MEO) that oxidizes unconverted anode fuel, preheats inlet air, recycles carbon dioxide (CO2) to the cathode, and reduces the pressure difference between the anode and cathode gas streams.
When water recovery and/or hydrogen or anode exhaust export is added to the fuel cell system, a blower is normally incorporated to offset the added pressure drop of the anode exhaust processing system, to pressure balance the anode and the cathode of the fuel cell. During upsets in the operation of the system (i.e., abnormal operation situations), for example, when the fuel cell system has a rapid change in power output or the blower malfunctions, a substantial pressure imbalance between the anode and the cathode of the fuel cell can occur. For example, upon a rapid reduction in power output a pressure imbalance can result from the fact that the steam and carbon dioxide flow from the fuel cell anode exhaust is instantaneously reduced. Meanwhile, the anode exhaust blower may take several seconds to reduce speed. During this time, there is insufficient flow relative to the blower speed, resulting in low anode pressure relative to cathode pressure (i.e., anode under-pressurization). On the other hand, if the blower speed is too slow, for example, due to loss of power to a speed controller of the blower or other blower or controller failure, then high anode pressure relative to cathode pressure will result (i.e., anode over-pressurization). A high pressure differential between the anode and the cathode may damage the seals of the fuel cell.
A conventional method of avoiding anode over-pressurization is to use a water seal that vents to atmosphere. In this case, to keep the same level of overpressure protection, the water level in the water seal must be adjusted according to changes in system pressure. If the water level is too low relative to system pressure, then process gases can escape through the water seal under non-upset, normal operating conditions. If the water level is too high relative to system pressure, then the water seal would lose the desired overpressure protection desired because the water seal would not activate until a greater than desired overpressure event occurred. These are the limitations of the conventional water seal that vents to atmosphere in preventing anode over-pressurization. Alternatively, another conventional method of avoiding anode over-pressurization is to use a relief valve that vents to atmosphere in place of the conventional water seal that vents to atmosphere. Compared to the water seal the relief valve may have advantages of simplicity and cost, but may be more difficult or less reliable to adjust the relief pressure compared to the water seal.
The use of a conventional water seal or relief valve that vents to atmosphere does not resolve the potential for anode under-pressurization. Furthermore, anode under-pressurization is the more common process upset than anode over-pressurization because it often occurs whenever the fuel cell drops load quickly, such as to zero power output. There are several mechanisms for the fuel cell to drop load quickly, for example, inverter fault or grid fluctuation which requires the inverter to shut down. Thus, when the anode under-pressure protection system is provided it is anticipated that it would be activated frequently.
Therefore, a need exists for improved technology that can be used to limit the extent of both over-pressurization and under-pressurization of the anode relative to the cathode, thereby reducing the risk of damage to the fuel cell. The system and method described in the examples below are configured to resolve anode under-pressurization protection by allowing a non-oxidizing gas to flow into the area of low pressure, thereby limiting the magnitude of the low pressure while also improving anode over-pressure protection.